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poly(styrene-butadiene-styrene) composite fiber for use as highly sensitive and flexible ammonia sensor
Synthetic Metals 233 (2017) 86–93
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Nanostructured polyaniline/poly(styrene-butadiene-styrene) composite fiber for use as highly sensitive and flexible ammonia sensor
MARK
Xingping Wanga, Si Menga, Mike Tebyetekerwaa, Wei Wenga, Jürgen Pionteckb, Bin Suna, ⁎ Zongyi Qina, Meifang Zhua, a State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, Shanghai 201620, PR China b Leibniz Institute of Polymer Research Dresden, Hohe Straße 6, 01069 Dresden, Germany
A R T I C L E I N F O
A B S T R A C T
Keyword: Conducting polymer Polyaniline Ammonia sensor Fiber based sensor Flexible electronics
Sensors with flexible and stretchable features are of great interest owing to their potential applications in flexible electronics. To this date, to fabricate gas sensors with both high sensitivity and flexibility is still a challenge. Herein, we have successfully coated thin film of polyaniline (PANI) onto flexible poly(styrene-butadiene-styrene) (SBS) fiber using in-situ dilute polymerization. The SBS/PANI composite fibers were evaluated as ammonia gas sensor. Morphological analysis of the nano-composite fiber revealed that the thin film consists of interconnected PANI nanofibers. Due to the high surface areas of the interconnected PANI nanofibers and the super flexibility of SBS fiber substrates, the SBS/PANI composite fiber based sensor demonstrated fascinating performance, including superior sensitivity (5.8 for 25 ppm, 16.4 for 100 ppm), fast response (≤13 s), good reproducibility as well as excellent mechanical reliability. Notably, the gas sensor is capable of sensing a concentration of ammonia gas as low as 0.1 ppm at room temperature. Based on its superior performance, its application in detecting ammonia gas leak is demonstrated, signifying its tremendous potential for applications in flexible electronics.
1. Introduction
nanofibers which exhibited significantly much better performance than conventional PANI films in both response sensitivity and rate. Rutledge et al. [24] also electrospun PANI nanofibers for chemiresistive gas sensors. The fibers showed excellent sensing performance to ammonia and nitrogen dioxide due to their ultra-high specific surface areas. Although the use of PANI nanofibers offers the prospect of high sensitivity and rapid response, the application of PANI nanofibers is limited by their poor mechanical strength and flexibility resulting partly from their rigid conjugated backbone structure and the fabrication techniques. As an alternative approach, flexible conductive composite materials were previously developed by coating PANI layers onto flexible polymeric films, which were expected to combine the sensing properties of PANI with the mechanical flexibility of polymeric substrates [25–27]. In-situ chemical polymerization can be used to form thin PANI layers on the surface of flexible polymeric substrates which can improve the effective surface areas to a certain degree. To cite an example, Patil et al. [17] developed a flexible polyester-PANI based ammonia sensor through in-situ polymerization, and the sensor exhibited a fast and stable response to ammonia. However, the sensing performance was still far less than that of the PANI nanofibers earlier mentioned. This
Ammonia is well recognized as hazardous industrial gas, which can cause serious problems to both environment and public health even at very low concentration [1,2]. Therefore, the detection of ammonia is an urgent need for numerous applications, including environmental monitoring [3,4], food industries [5,6], and more recently in biomedical diagnosis [7,8]. So far, numerous ammonia sensors have been developed, based on inorganic metal oxides [9], hybrid materials [10–12], and conducting polymers [13,14]. Among which, conducting polymers may be a potential alternative owing to some advantages such as ease of synthesis, processing, modifiable electrical conductivity, and operability at room temperature [15–17]. In particular polyaniline (PANI) has been extensively studied as an efficient ammonia gas sensing material due to its unique doping/dedoping chemistry, stable electrical conduction and good environmental stability [18–21]. One-dimensional (1D) nanostructured forms of PANI, such as nanofibers and nanorods have gained exceptional attention due to their high surface area and porosity which allows for rapid diffusion of analytes into and out of the structures [22]. For example, Virji et al. [23] employed interfacial polymerization method to synthesize PANI
⁎
Corresponding author. E-mail address: [email protected] (M. Zhu).
http://dx.doi.org/10.1016/j.synthmet.2017.09.012 Received 28 July 2017; Received in revised form 18 September 2017; Accepted 22 September 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.
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Electronic Instrument Co., Ltd., China).
might be attributed to the formation of regular PANI particles instead of interconnected PANI nanofibers by the conventional in-situ polymerization [23]. Thus, the development of flexible ammonia sensors with both high sensitivity and good mechanical flexibility remains a big challenge. In this work, we have successfully coated interconnected nanofibers of PANI onto flexible poly(styrene-butadiene-styrene) (SBS) fiber using the in-situ dilute polymerization method. Such nanostructured PANI/ SBS composite fiber not only possess high active surface areas but facilitate easy access of target gas chemicals as well as high flexibility. Due to the unique structure, the composite fiber exhibited remarkable sensitivity (5.8 for 25 ppm, 16.4 for 100 ppm), fast response (≤13 s), good reproducibility as well as excellent mechanical reliability. In particular, the gas sensor possesses an ultra-low detection limit at 0.1 ppm of ammonia gas at room temperature. Additionally, the SBS/ PANI fiber was integrated into a commercial respirator and connected with electronic components to demonstrate practical application in detecting ammonia gas.
2.4. Sensing characterization To investigate the sensitivity, reversibility, and reproducibility of the flexible PANI fibers upon exposure to ammonia, the fiber was placed in a 1 L chamber with a gas inlet/outlet maintained at a standard atmospheric pressure. The corresponding amount of ammonia was injected into the chamber using micro-injector. The sensor was first exposed to ammonia for a period of time, then the sensors was removed from the chamber and exposed to air. This process was repeatedly performed for several times. The resistance changes of SBS/PANI fibers were simultaneously monitored with a Keithley 6487 picoammeter. All sensing measurements were carried out at room temperature (20 °C) and 65% relative humidity. The responses of the flexible SBS/PANI sensor are reported as ΔR/ R0, where ΔR = Rex − R0, R0 is the initial resistance prior to any exposure to ammonia, and Rex is the maximum resistance upon exposure to ammonia. The response time is defined as the time required for the change in signal to reach within 1/e (e is the EULER number) of maximum resistance value in presence of ammonia [24].
2. Experimental 2.1. Materials and reagents
3. Results and discussions Commercially available SBS D1102 K triblock copolymer with a butadiene/styrene weight ratio of 72/28 and a density of 0.94 g cm−3 was purchased from Kraton, USA. Aniline, ammonium persulfate (APS), and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co. Ltd., China. All reagents were of analytical grade and used as received without further purification. Ammonia (purity: 99.99%) gas was purchased from Shanghai Chenggong gas Co. Ltd.
3.1. Structure and morphology The cross section of solution-spun fibers has an irregular nonspherical flattened shape in dimension of ca. 50 μm times 250 μm with a smooth surface (Fig. S2, Supplementary information), as shown in Fig. 1a. The SBS/PANI fibers were fabricated though in situ polymerization of aniline on the surface of pure SBS fiber. The surface and cross-section SEM images of SBS/PANI fibers prepared with different concentration of aniline are shown in Fig. 1b-j. As can be seen from Fig. 1b, the SBS fiber is uniformly covered by a thin PANI layer, indicating a skin-core structure. It is observed that the lower concentration of aniline (0.01 M) has a higher likelihood to form interconnected PANI nanofibers (Fig. 1c, Fig. S3) than the polymerization starting from higher concentration where dense PANI nanoparticles, rather than interconnected fibers, were observed (Fig. 1e, g, i). For the sample of SBS/ PANI0.01M, a top view of the SBS/PANI fiber shows highly uniform interconnected PANI nanofibers, and the diameters of the nanofibers ranged from 20 nm to 50 nm (Fig. 1c). Moreover, close inspection of the cross-sectional morphologies showed that the PANI nanofibers were rooted on the surface of the flexible SBS fibers. The adhesion between the SBS fiber and PANI is believed to have originated from the strong ππ* stacking interaction of phenyl groups of SBS and PANI [30]. Only at thick layers delamination of the PANI layer from the fiber matrix appears during cryo-fracturing (Figs. 1b, j). The morphological evolution of deposited PANI film can be explained by using the classical theory of nucleation and growth, that is, PANI morphology depends on the mechanism of nucleation followed by growth. Elongated form (e.g., fiber or rods) is established as the growth rate for PANI is distinctly not identical in all directions (anisotropic growth). According to the mechanism proposed by Chiou [28], there are two possible nucleation sites during the process of in situ polymerization of aniline. Bulk solution and solid substrates, and these two sites compete. More specifically, when very dilute aniline/oxidant is used, the nanofibrils are initially formed on solid substrates, and those nanofibrils can continuously grow and elongate in one direction to form nanofibers. Further, in dilute polymerization, a larger amount of polyaniline can be continuously deposited onto the active nuclei resulting in interconnected 3D network structures of PANI nanofibers. In contrast, when concentrated aniline/oxidant is used, the PANI nanofibrils formation rate in bulk solution will be similar or faster than deposition rate on solid substrates [28]. A large number of PANI nanofibrils form in bulk solution and precipitate on solid substrates, then
2.2. Sample preparation Flexible SBS fibers were fabricated by wet spinning (see Supporting information for detailed descriptions). In-situ dilute polymerization was used to grow PANI layers on the surface of flexible SBS fibers using APS as oxidant and 1 M hydrochloric acid as dopant [28]. Typically, 0.1 g SBS fibers were immersed into 50 mL aniline/1 M dopant acid solution for 1 h at room temperature and then APS/1 M dopant acid solution was quickly added. The polymerization was carried out without disturbance for 6 h at 0–5 °C temperature. The molar ratio of aniline monomer to APS was kept at 1, and the initial concentrations of aniline/APS based on the total volume of reaction mixture were varied from 0.01 M to 0.1 M. Finally, the fibers were washed with a large excess of aq. HCl (M) under ultrasonic treatment for 10 min (SK1200H, with the power of 50 W and working frequency of 53 kHz, Shanghai Kudos Ultrasonic Instrument Co. Ltd., China) and dried in vacuum at room temperature for at least 48 h at which time the resistance was constant [29]. In this paper, samples are denoted as SBS/PANIx for simplification, where x represents the initial concentration of aniline. For instance, SBS/PANI0.01M represents the composite fiber prepared with aniline concentration of 0.01 M. 2.3. Structural and mechanical characterization The surface and cross-sectional (fractured in liquid nitrogen) morphologies of the samples were characterized by field-emission scanning electron microscopy using a JEOL JSM-4800LV. The samples for SEM observation were sputter coated with platinum. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of pure SBS fiber and SBS/PANI composite fiber were recorded on Nicolet 6700 FT-IR spectrophotometer with variable angle horizontal ATR accessory, on which a 45° rectangle ZnSe crystal was used. Tensile measurements were carried out at a constant tensile velocity of 20 mm min−1 with a gauge length of 5 mm on a fiber tension tester (Changzhou Dahua 87
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Fig. 1. Cross-section SEM image of (a) Pure SBS fiber and (b) SBS/PANI0.1M fiber. Surface SEM image of (c) SBS/ PANI0.01M, (e) SBS/PANI0.03M, (g) SBS/PANI0.05M, and (i) SBS/PANI0.1M. Cross-section SEM image of (d) SBS/ PANI0.01M, (f) SBS/PANI0.03M, (h) SBS/PANI0.05M, and (j) SBS/PANI0.1M.
PANI0.1M are 90–150 nm, 150–200 nm, 180–250 nm, and 200–400 nm, respectively. As observed, the thickness of PANI layer increases with the concentration of aniline. It is generally known that the surface areas and thickness of sensing materials would have great effect on the
simultaneously followed by secondary nucleation and growth, and eventually grows into irregular particles [31]. The cross-sectional SEM images in Fig. 1 show that the PANI sheath thickness of SBS/PANI0.01M, SBS/PANI0.03M, SBS/PANI0.05M, and SBS/ 88
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PANI fibers upon ammonia exposure are reasonably reversible and reproducible. The maximum ΔR/R0 value was almost unchanged over multiple cycles of exposure to the same concentration of ammonia, demonstrating that the SBS/PANI fibers can be used repeatedly for ammonia sensing. After the ammonia gas was replaced by dry air flow, the resistance of SBS/PANI fiber quickly decreased, but could not fully return to the initial value due to the residual NH4+ ions in the PANI backbone which inflicts the irreversible change of conductance. This instability of sensor response is called “drift”, such phenomenon was also observed in previous reports [24,39,40]. In addition to responsivity and applicability, a good gas sensor is also expected to provide fast response to the analyte. The response and recovery times of the as-prepared SBS/PANI fibers upon exposure to 25 ppm and 100 ppm of ammonia are shown in Fig. 3d. The SBS/PANI fibers showed a very fast response (≤13 s) upon cyclic exposure to ammonia/air. Higher ammonia concentration gave the fastest response. This can be attributed to the quick attack of a larger number of ammonia molecules at the sensor surface in a short time [25]. Furthermore, the selectivity of the SBS/PANI fiber ammonia sensors was investigated. The flexible SBS/PANI fiber sensors were tested for different volatile organic vapors and gases including ammonia, hydrogen chloride, nitrogen dioxide, acetic acid, cyclohexane, trichloromethane, ethanol, and carbon dioxide as shown in Fig. 4. It is noted that the response to ammonia is significantly higher than these of hydrogen chloride, nitrogen dioxide, acetic acid, cyclohexane, trichloromethane, ethanol, and carbon dioxide. These results indicate that the flexible SBS/PANI fiber sensors exhibit effective selectivity. SBS/PANI0.01M fiber was successively exposed to different ammonia concentration ranging from 100 ppm down to 0.1 ppm (Fig. 5a). A stepwise decrease in response was observed with a decrease in ammonia gas concentration in the range of 100 to 0.1 ppm. Furthermore, the response approached its original baseline level as soon as the sensor was exposed to dry air during each recovery phase, indicating the excellent reversibility of the flexible fiber-based sensor. It is worth noting that the flexible fiber-based sensor was capable of sensing a concentration of ammonia gas to as low as 0.1 ppm at room temperature. These results are comparable with that of the single PANI nanowire sensor [20] and much lower than that of most of the ammonia sensors [41]. The ammonia exposure limit in UK is 25 ppm over an 8-h period and 35 ppm over a 10 min period [42]. More importantly, the sensitivity of our SBS/PANI fiber fits well in these specifications. A plot of the variation in the sensor response as a function of ammonia gas concentration is shown in Fig. 5b. A good linear relationship between the responses (ΔR/R0) and ammonia concentrations from the low 10 ppm level to the high 100 ppm level was observed, which is essential for quantitative measurements of ammonia gas concentration. Results showed that the SBS/PANI0.01M fiber based sensor exhibited higher response and faster response compared to the PANI-based sensors reported in the previous literature, see Fig. 6 and Table S1 [17,24,25,39,43–47]. The excellent sensing performance can be attributed to the high surface areas and porosity of the interconnected nanofibers structure as mentioned previously. As a result, ammonia can rapidly diffuse into and out of PANI layers which result in faster response and recovery. The ammonia gas sensing mechanism can be explained by the interaction between the PANI sensitive layer and the ammonia gas molecules. The speculated reaction mechanism in the ammonia gas sensing process is schematically represented in Fig. 7. It is well known that doping and dedoping play key roles in the sensing mechanism of PANIbased sensors [13]. The conductivity of pure PANI is rather low (< 10−5 S cm−1). To achieve highly conductive PANI, the doping process is necessary. In this work, HCl doped PANI, the emeraldine salt, was used as ammonia sensor. Due to the doping reaction, neutral PANI molecules gain protons (H+), forming energetically favorable N+-H chemical bonds, resulting in an increased number of charge carriers, and therefore, the resistance of PANI decreased. PANI doping process is
Fig. 2. ATR-FTIR spectra of SBS fiber and SBS/PANI0.01M fiber.
sensitivity and response time [32]. Therefore, the SBS/PANI0.01M composite fiber is expected to have the highest sensitivity and fastest response time because of the dramatically increased surface areas, which are derived from the 3D architecture of interconnected PANI nanofibers compared to the composites fibers with higher PANI amounts. 3.2. FTIR spectral analysis In the FTIR spectrum of SBS fiber, see curve A in Fig. 2, the following peaks can be identified: 3060, 3024, 756, and 699 cm−1 which are related to CeH stretching of the benzene ring side chain of styrene units [33]. The characteristic absorption peaks at 912, 966, 1601, and 3006 cm−1 are due to the double bonds of polybutadiene units, and the absorption peaks at 2917, 2844 cm−1 attributed to the CeH stretching of butadiene [34]. The FTIR spectrum of SBS/PANI composite fiber exhibits almost all the bands present in SBS fiber, see curve B in Fig. 2. In addition, new bands at 3226, 1308, 1242, and 1154 cm−1 are typical of the pernigraniline form of PANI. The broad band at 3226 cm−1 was assigned to the symmetric and asymmetric vibrational modes of the NH2 group [35]. The absorption band at 1311 cm−1 corresponds to πelectron delocalization induced in PANI by protonation [36]. The band characteristic of the conducting protonated form is observed at 1242 cm−1 and interpreted as a CeN+ stretching vibration in the polaron structure [37]. The prominent 1154 cm−1 band is assigned to a vibration mode of the eNH%+ structure, which is formed during protonation [38]. These entire characteristic bands confirm the formation of conducting PANI. 3.3. Ammonia gas sensing properties and mechanism For rendering an ammonia sensor highly suitable for application, both high responsivity and applicability are key features. Therefore, the as-prepared SBS/PANI fibers were exposed to repeated cycles 100 s to 25 or 100 ppm of ammonia followed by air purging at room temperature. The obtained results are given in Fig. 3, Fig. S4 and Fig. S5 (Supplementary information). A sharp increase in electrical resistance was observed when the SBS/PANI fibers were exposed to ammonia gas (Fig. 3). Responses as large as ΔR/R0 = 5.8 ± 0.3, 2.2 ± 0.2, 2.0 ± 0.1, and 2.0 ± 0.2 were observed at 25 ppm ammonia for SBS/ PANI0.01M fibers, SBS/PANI0.03M fibers, SBS/PANI0.05M fibers, and SBS/ PANI0.1M fibers, respectively (Fig. 3c). Therefore, as expected, SBS/ PANI0.01M fibers with PANI nano-layers consisting of interconnected nanofibers showed the highest response (ΔR/R0 = 5.8 ± 0.3 at 25 ppm and ΔR/R0 = 16.4 ± 0.8 at 100 ppm) to ammonia due to their high surface areas, small nanofiber diameter, and porous structure. As expected, with higher analyte concentration the response intensity increases. For applicability, Fig. 3 also indicates that the responses of SBS/ 89
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Fig. 3. Response of SBS/PANI0.01M fibers under cyclic exposure to (a) 25 and (b) 100 ppm of ammonia. (c) maximum response for the SBS/PANI fibers to 25 and 100 ppm ammonia in dependence on the composition. (d) Response time (averaged over at least three cycles) of SBS/PANI fibers fabricated at various aniline concentration during exposure of 25 and 100 ppm of ammonia.
Fig. 4. Response of the SBS/PANI0.01M fiber to ammonia (100 ppm) and different diluted organic vapors (concentration = 1%). Fig. 6. Comparison of sensitivity and response time of different PANI-based sensors for ammonia at 100 ppm.
reversible, that is, the doped PANI can be turned into its undoped state by chemical reactions. When PANI was exposed to ammonia gas, ammonia molecules diffused into PANI layer and took up protons from PANI, forming a more energetically favorable ammonium (NH4+) and the emeraldine base. This resulted in a decreased number of charge carriers, i.e., decreasing the doping level of PANI. As a result, the resistance of the PANI sharply increased [25]. This process is reversible, when the PANI sensor is exposed to air, the ammonium ion decomposes into ammonia and H+, then the H+ is captured by PANI, which restore its initial doping level and cause an increase in the sensor conductivity, approaching the initial resistance [17].
3.4. Mechanical reliability of SBS/PANI composite fiber Excellent mechanical reliability is expected to be one of the most important properties of fiber-based sensors for their real applications in flexible electronics. Fig. 8a shows the representative strain-stress curves of pure SBS fiber and SBS/PANI composite fiber. The fibers exhibited similar tensile behaviors as elastomeric materials. Good deformation capabilities and a maximum strain of up to 1000% is shown by the fibers, demonstrating their excellent flexibility. To evaluate the Fig. 5. (a) Response of the SBS/PANI0.01M fiber exposed to ammonia gas at different concentrations. (b) The sensor responses as a function of ammonia gas concentration from 10 ppm to 100 ppm.
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micro-injector, a few seconds later, the LED light turned on, and the buzzer gave an alarm (see Video S2 and Figure S7), demonstrating the practical applications of the developed SBS/PANI fiber based ammonia sensor for flexible electronics. A very important point to note is that according to the Chinese Indoor Air Quality Standard GB/T18883-2002 the maximum allowable concentration of ammonia is 0.2 ppm. When ammonia concentration reaches ∼5.3 ppm, it can be smelled. However, a person wearing the gas mask will not smell ammonia at this concentration. Therefore, this makes our fiber sensor applicable, as it would easily detect ammonia concentration of at least 1 ppm under masking conditions of the wearer. Fig. 7. Reaction mechanism in the ammonia gas sensing process.
4. Conclusions electrical reliability of the SBS/PANI composite fiber under bending deformations, a cyclic bending-straightening test was conducted using a homemade two-point bending device (see Video S1). The results of bending testing show that the resistance remains almost unchanged even after 1000 bending cycles, demonstrating the excellent electrical stability of the SBS/PANI fiber (Fig. 8b). Sensing behaviors in both straight and bent configurations were investigated during exposure to 25 ppm ammonia gas at room temperature (Fig. 8c). It is clearly observed that twisted SBS/PANI0.01M fiber shows a similar response to that of straight SBS/PANI0.01M fiber owing to the bendable and flexible natures of SBS fiber. These results indicate that the fibers had excellent mechanical reliability.
In conclusion, we reported a gas sensor based on a flexible SBS/ PANI fiber, fabricated through a facile process, which demonstrated superior performance for detection of ammonia gas. Owing to the high surface areas, small diameters, and porous structure, the response sensitivity of interconnected nanofibers was significantly better than those of nano-layers with irregular particles. The flexible SBS/ PANI0.01M fiber revealed several remarkable features including high sensitivity (5.8 for 25 ppm, 16.4 for 100 ppm), fast response (≤13 s), good reproducibility, ultra-low detection limit (0.1 ppm) as well as excellent mechanical reliability. Furthermore, we developed a flexible gas sensing and alarm system based on SBS/PANI fibers, demonstrating its tremendous potential applications in flexible electronics.
3.5. Development of a flexible SBS/PANI fiber based sensor for detection of ammonia gas
Acknowledgements
The SBS/PANI0.01M fiber was integrated into a commercial respirator and connected with electronic components to develop an ammonia gas sensor device with a LED light and a buzzer alarm (Fig. 9). The block diagram of the sensor circuit and optical image of the circuit board are shown in Fig. S6. The developed ammonia gas sensing respirator was put on a head model and placed in a 40 L chamber. Subsequently, 1 mL ammonia gas was injected into the chamber using
This work was supported by Shanghai Fundamental Research Projects (Project No. 16JC1400701), Program for Changjiang Scholars and Innovative Research Team in University (IRT16R13) and Opening Project of State Key Laboratory of for Modification of Chemical Fibers and Polymer Materials, Donghua University (Contract Number: LK1607). J.P. appreciates the support by the SKLPMF Visiting Scholar Funding Project Number LK1607. Fig. 8. (a) The representative stress-strain curves of pure SBS fiber and SBS/PANI fiber. (b) Resistance stability of SBS/PANI composite fiber during bending for 1000 times. (c) Gas response of straight and twisted SBS/PANI0.01M fiber exposed to ammonia at 25 ppm. (d) Photo of the straight and twisted test geometry.
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Fig. 9. Optical images of (a) SBS/PANI fiber and (b) commercial respirator with ammonia gas sensor.
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Nanostructured polyaniline/poly(styrene-butadiene-styrene) composite fiber for use as highly sensitive and flexible ammonia sensor
MARK
Xingping Wanga, Si Menga, Mike Tebyetekerwaa, Wei Wenga, Jürgen Pionteckb, Bin Suna, ⁎ Zongyi Qina, Meifang Zhua, a State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, Shanghai 201620, PR China b Leibniz Institute of Polymer Research Dresden, Hohe Straße 6, 01069 Dresden, Germany
A R T I C L E I N F O
A B S T R A C T
Keyword: Conducting polymer Polyaniline Ammonia sensor Fiber based sensor Flexible electronics
Sensors with flexible and stretchable features are of great interest owing to their potential applications in flexible electronics. To this date, to fabricate gas sensors with both high sensitivity and flexibility is still a challenge. Herein, we have successfully coated thin film of polyaniline (PANI) onto flexible poly(styrene-butadiene-styrene) (SBS) fiber using in-situ dilute polymerization. The SBS/PANI composite fibers were evaluated as ammonia gas sensor. Morphological analysis of the nano-composite fiber revealed that the thin film consists of interconnected PANI nanofibers. Due to the high surface areas of the interconnected PANI nanofibers and the super flexibility of SBS fiber substrates, the SBS/PANI composite fiber based sensor demonstrated fascinating performance, including superior sensitivity (5.8 for 25 ppm, 16.4 for 100 ppm), fast response (≤13 s), good reproducibility as well as excellent mechanical reliability. Notably, the gas sensor is capable of sensing a concentration of ammonia gas as low as 0.1 ppm at room temperature. Based on its superior performance, its application in detecting ammonia gas leak is demonstrated, signifying its tremendous potential for applications in flexible electronics.
1. Introduction
nanofibers which exhibited significantly much better performance than conventional PANI films in both response sensitivity and rate. Rutledge et al. [24] also electrospun PANI nanofibers for chemiresistive gas sensors. The fibers showed excellent sensing performance to ammonia and nitrogen dioxide due to their ultra-high specific surface areas. Although the use of PANI nanofibers offers the prospect of high sensitivity and rapid response, the application of PANI nanofibers is limited by their poor mechanical strength and flexibility resulting partly from their rigid conjugated backbone structure and the fabrication techniques. As an alternative approach, flexible conductive composite materials were previously developed by coating PANI layers onto flexible polymeric films, which were expected to combine the sensing properties of PANI with the mechanical flexibility of polymeric substrates [25–27]. In-situ chemical polymerization can be used to form thin PANI layers on the surface of flexible polymeric substrates which can improve the effective surface areas to a certain degree. To cite an example, Patil et al. [17] developed a flexible polyester-PANI based ammonia sensor through in-situ polymerization, and the sensor exhibited a fast and stable response to ammonia. However, the sensing performance was still far less than that of the PANI nanofibers earlier mentioned. This
Ammonia is well recognized as hazardous industrial gas, which can cause serious problems to both environment and public health even at very low concentration [1,2]. Therefore, the detection of ammonia is an urgent need for numerous applications, including environmental monitoring [3,4], food industries [5,6], and more recently in biomedical diagnosis [7,8]. So far, numerous ammonia sensors have been developed, based on inorganic metal oxides [9], hybrid materials [10–12], and conducting polymers [13,14]. Among which, conducting polymers may be a potential alternative owing to some advantages such as ease of synthesis, processing, modifiable electrical conductivity, and operability at room temperature [15–17]. In particular polyaniline (PANI) has been extensively studied as an efficient ammonia gas sensing material due to its unique doping/dedoping chemistry, stable electrical conduction and good environmental stability [18–21]. One-dimensional (1D) nanostructured forms of PANI, such as nanofibers and nanorods have gained exceptional attention due to their high surface area and porosity which allows for rapid diffusion of analytes into and out of the structures [22]. For example, Virji et al. [23] employed interfacial polymerization method to synthesize PANI
⁎
Corresponding author. E-mail address: [email protected] (M. Zhu).
http://dx.doi.org/10.1016/j.synthmet.2017.09.012 Received 28 July 2017; Received in revised form 18 September 2017; Accepted 22 September 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.
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Electronic Instrument Co., Ltd., China).
might be attributed to the formation of regular PANI particles instead of interconnected PANI nanofibers by the conventional in-situ polymerization [23]. Thus, the development of flexible ammonia sensors with both high sensitivity and good mechanical flexibility remains a big challenge. In this work, we have successfully coated interconnected nanofibers of PANI onto flexible poly(styrene-butadiene-styrene) (SBS) fiber using the in-situ dilute polymerization method. Such nanostructured PANI/ SBS composite fiber not only possess high active surface areas but facilitate easy access of target gas chemicals as well as high flexibility. Due to the unique structure, the composite fiber exhibited remarkable sensitivity (5.8 for 25 ppm, 16.4 for 100 ppm), fast response (≤13 s), good reproducibility as well as excellent mechanical reliability. In particular, the gas sensor possesses an ultra-low detection limit at 0.1 ppm of ammonia gas at room temperature. Additionally, the SBS/ PANI fiber was integrated into a commercial respirator and connected with electronic components to demonstrate practical application in detecting ammonia gas.
2.4. Sensing characterization To investigate the sensitivity, reversibility, and reproducibility of the flexible PANI fibers upon exposure to ammonia, the fiber was placed in a 1 L chamber with a gas inlet/outlet maintained at a standard atmospheric pressure. The corresponding amount of ammonia was injected into the chamber using micro-injector. The sensor was first exposed to ammonia for a period of time, then the sensors was removed from the chamber and exposed to air. This process was repeatedly performed for several times. The resistance changes of SBS/PANI fibers were simultaneously monitored with a Keithley 6487 picoammeter. All sensing measurements were carried out at room temperature (20 °C) and 65% relative humidity. The responses of the flexible SBS/PANI sensor are reported as ΔR/ R0, where ΔR = Rex − R0, R0 is the initial resistance prior to any exposure to ammonia, and Rex is the maximum resistance upon exposure to ammonia. The response time is defined as the time required for the change in signal to reach within 1/e (e is the EULER number) of maximum resistance value in presence of ammonia [24].
2. Experimental 2.1. Materials and reagents
3. Results and discussions Commercially available SBS D1102 K triblock copolymer with a butadiene/styrene weight ratio of 72/28 and a density of 0.94 g cm−3 was purchased from Kraton, USA. Aniline, ammonium persulfate (APS), and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co. Ltd., China. All reagents were of analytical grade and used as received without further purification. Ammonia (purity: 99.99%) gas was purchased from Shanghai Chenggong gas Co. Ltd.
3.1. Structure and morphology The cross section of solution-spun fibers has an irregular nonspherical flattened shape in dimension of ca. 50 μm times 250 μm with a smooth surface (Fig. S2, Supplementary information), as shown in Fig. 1a. The SBS/PANI fibers were fabricated though in situ polymerization of aniline on the surface of pure SBS fiber. The surface and cross-section SEM images of SBS/PANI fibers prepared with different concentration of aniline are shown in Fig. 1b-j. As can be seen from Fig. 1b, the SBS fiber is uniformly covered by a thin PANI layer, indicating a skin-core structure. It is observed that the lower concentration of aniline (0.01 M) has a higher likelihood to form interconnected PANI nanofibers (Fig. 1c, Fig. S3) than the polymerization starting from higher concentration where dense PANI nanoparticles, rather than interconnected fibers, were observed (Fig. 1e, g, i). For the sample of SBS/ PANI0.01M, a top view of the SBS/PANI fiber shows highly uniform interconnected PANI nanofibers, and the diameters of the nanofibers ranged from 20 nm to 50 nm (Fig. 1c). Moreover, close inspection of the cross-sectional morphologies showed that the PANI nanofibers were rooted on the surface of the flexible SBS fibers. The adhesion between the SBS fiber and PANI is believed to have originated from the strong ππ* stacking interaction of phenyl groups of SBS and PANI [30]. Only at thick layers delamination of the PANI layer from the fiber matrix appears during cryo-fracturing (Figs. 1b, j). The morphological evolution of deposited PANI film can be explained by using the classical theory of nucleation and growth, that is, PANI morphology depends on the mechanism of nucleation followed by growth. Elongated form (e.g., fiber or rods) is established as the growth rate for PANI is distinctly not identical in all directions (anisotropic growth). According to the mechanism proposed by Chiou [28], there are two possible nucleation sites during the process of in situ polymerization of aniline. Bulk solution and solid substrates, and these two sites compete. More specifically, when very dilute aniline/oxidant is used, the nanofibrils are initially formed on solid substrates, and those nanofibrils can continuously grow and elongate in one direction to form nanofibers. Further, in dilute polymerization, a larger amount of polyaniline can be continuously deposited onto the active nuclei resulting in interconnected 3D network structures of PANI nanofibers. In contrast, when concentrated aniline/oxidant is used, the PANI nanofibrils formation rate in bulk solution will be similar or faster than deposition rate on solid substrates [28]. A large number of PANI nanofibrils form in bulk solution and precipitate on solid substrates, then
2.2. Sample preparation Flexible SBS fibers were fabricated by wet spinning (see Supporting information for detailed descriptions). In-situ dilute polymerization was used to grow PANI layers on the surface of flexible SBS fibers using APS as oxidant and 1 M hydrochloric acid as dopant [28]. Typically, 0.1 g SBS fibers were immersed into 50 mL aniline/1 M dopant acid solution for 1 h at room temperature and then APS/1 M dopant acid solution was quickly added. The polymerization was carried out without disturbance for 6 h at 0–5 °C temperature. The molar ratio of aniline monomer to APS was kept at 1, and the initial concentrations of aniline/APS based on the total volume of reaction mixture were varied from 0.01 M to 0.1 M. Finally, the fibers were washed with a large excess of aq. HCl (M) under ultrasonic treatment for 10 min (SK1200H, with the power of 50 W and working frequency of 53 kHz, Shanghai Kudos Ultrasonic Instrument Co. Ltd., China) and dried in vacuum at room temperature for at least 48 h at which time the resistance was constant [29]. In this paper, samples are denoted as SBS/PANIx for simplification, where x represents the initial concentration of aniline. For instance, SBS/PANI0.01M represents the composite fiber prepared with aniline concentration of 0.01 M. 2.3. Structural and mechanical characterization The surface and cross-sectional (fractured in liquid nitrogen) morphologies of the samples were characterized by field-emission scanning electron microscopy using a JEOL JSM-4800LV. The samples for SEM observation were sputter coated with platinum. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of pure SBS fiber and SBS/PANI composite fiber were recorded on Nicolet 6700 FT-IR spectrophotometer with variable angle horizontal ATR accessory, on which a 45° rectangle ZnSe crystal was used. Tensile measurements were carried out at a constant tensile velocity of 20 mm min−1 with a gauge length of 5 mm on a fiber tension tester (Changzhou Dahua 87
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Fig. 1. Cross-section SEM image of (a) Pure SBS fiber and (b) SBS/PANI0.1M fiber. Surface SEM image of (c) SBS/ PANI0.01M, (e) SBS/PANI0.03M, (g) SBS/PANI0.05M, and (i) SBS/PANI0.1M. Cross-section SEM image of (d) SBS/ PANI0.01M, (f) SBS/PANI0.03M, (h) SBS/PANI0.05M, and (j) SBS/PANI0.1M.
PANI0.1M are 90–150 nm, 150–200 nm, 180–250 nm, and 200–400 nm, respectively. As observed, the thickness of PANI layer increases with the concentration of aniline. It is generally known that the surface areas and thickness of sensing materials would have great effect on the
simultaneously followed by secondary nucleation and growth, and eventually grows into irregular particles [31]. The cross-sectional SEM images in Fig. 1 show that the PANI sheath thickness of SBS/PANI0.01M, SBS/PANI0.03M, SBS/PANI0.05M, and SBS/ 88
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PANI fibers upon ammonia exposure are reasonably reversible and reproducible. The maximum ΔR/R0 value was almost unchanged over multiple cycles of exposure to the same concentration of ammonia, demonstrating that the SBS/PANI fibers can be used repeatedly for ammonia sensing. After the ammonia gas was replaced by dry air flow, the resistance of SBS/PANI fiber quickly decreased, but could not fully return to the initial value due to the residual NH4+ ions in the PANI backbone which inflicts the irreversible change of conductance. This instability of sensor response is called “drift”, such phenomenon was also observed in previous reports [24,39,40]. In addition to responsivity and applicability, a good gas sensor is also expected to provide fast response to the analyte. The response and recovery times of the as-prepared SBS/PANI fibers upon exposure to 25 ppm and 100 ppm of ammonia are shown in Fig. 3d. The SBS/PANI fibers showed a very fast response (≤13 s) upon cyclic exposure to ammonia/air. Higher ammonia concentration gave the fastest response. This can be attributed to the quick attack of a larger number of ammonia molecules at the sensor surface in a short time [25]. Furthermore, the selectivity of the SBS/PANI fiber ammonia sensors was investigated. The flexible SBS/PANI fiber sensors were tested for different volatile organic vapors and gases including ammonia, hydrogen chloride, nitrogen dioxide, acetic acid, cyclohexane, trichloromethane, ethanol, and carbon dioxide as shown in Fig. 4. It is noted that the response to ammonia is significantly higher than these of hydrogen chloride, nitrogen dioxide, acetic acid, cyclohexane, trichloromethane, ethanol, and carbon dioxide. These results indicate that the flexible SBS/PANI fiber sensors exhibit effective selectivity. SBS/PANI0.01M fiber was successively exposed to different ammonia concentration ranging from 100 ppm down to 0.1 ppm (Fig. 5a). A stepwise decrease in response was observed with a decrease in ammonia gas concentration in the range of 100 to 0.1 ppm. Furthermore, the response approached its original baseline level as soon as the sensor was exposed to dry air during each recovery phase, indicating the excellent reversibility of the flexible fiber-based sensor. It is worth noting that the flexible fiber-based sensor was capable of sensing a concentration of ammonia gas to as low as 0.1 ppm at room temperature. These results are comparable with that of the single PANI nanowire sensor [20] and much lower than that of most of the ammonia sensors [41]. The ammonia exposure limit in UK is 25 ppm over an 8-h period and 35 ppm over a 10 min period [42]. More importantly, the sensitivity of our SBS/PANI fiber fits well in these specifications. A plot of the variation in the sensor response as a function of ammonia gas concentration is shown in Fig. 5b. A good linear relationship between the responses (ΔR/R0) and ammonia concentrations from the low 10 ppm level to the high 100 ppm level was observed, which is essential for quantitative measurements of ammonia gas concentration. Results showed that the SBS/PANI0.01M fiber based sensor exhibited higher response and faster response compared to the PANI-based sensors reported in the previous literature, see Fig. 6 and Table S1 [17,24,25,39,43–47]. The excellent sensing performance can be attributed to the high surface areas and porosity of the interconnected nanofibers structure as mentioned previously. As a result, ammonia can rapidly diffuse into and out of PANI layers which result in faster response and recovery. The ammonia gas sensing mechanism can be explained by the interaction between the PANI sensitive layer and the ammonia gas molecules. The speculated reaction mechanism in the ammonia gas sensing process is schematically represented in Fig. 7. It is well known that doping and dedoping play key roles in the sensing mechanism of PANIbased sensors [13]. The conductivity of pure PANI is rather low (< 10−5 S cm−1). To achieve highly conductive PANI, the doping process is necessary. In this work, HCl doped PANI, the emeraldine salt, was used as ammonia sensor. Due to the doping reaction, neutral PANI molecules gain protons (H+), forming energetically favorable N+-H chemical bonds, resulting in an increased number of charge carriers, and therefore, the resistance of PANI decreased. PANI doping process is
Fig. 2. ATR-FTIR spectra of SBS fiber and SBS/PANI0.01M fiber.
sensitivity and response time [32]. Therefore, the SBS/PANI0.01M composite fiber is expected to have the highest sensitivity and fastest response time because of the dramatically increased surface areas, which are derived from the 3D architecture of interconnected PANI nanofibers compared to the composites fibers with higher PANI amounts. 3.2. FTIR spectral analysis In the FTIR spectrum of SBS fiber, see curve A in Fig. 2, the following peaks can be identified: 3060, 3024, 756, and 699 cm−1 which are related to CeH stretching of the benzene ring side chain of styrene units [33]. The characteristic absorption peaks at 912, 966, 1601, and 3006 cm−1 are due to the double bonds of polybutadiene units, and the absorption peaks at 2917, 2844 cm−1 attributed to the CeH stretching of butadiene [34]. The FTIR spectrum of SBS/PANI composite fiber exhibits almost all the bands present in SBS fiber, see curve B in Fig. 2. In addition, new bands at 3226, 1308, 1242, and 1154 cm−1 are typical of the pernigraniline form of PANI. The broad band at 3226 cm−1 was assigned to the symmetric and asymmetric vibrational modes of the NH2 group [35]. The absorption band at 1311 cm−1 corresponds to πelectron delocalization induced in PANI by protonation [36]. The band characteristic of the conducting protonated form is observed at 1242 cm−1 and interpreted as a CeN+ stretching vibration in the polaron structure [37]. The prominent 1154 cm−1 band is assigned to a vibration mode of the eNH%+ structure, which is formed during protonation [38]. These entire characteristic bands confirm the formation of conducting PANI. 3.3. Ammonia gas sensing properties and mechanism For rendering an ammonia sensor highly suitable for application, both high responsivity and applicability are key features. Therefore, the as-prepared SBS/PANI fibers were exposed to repeated cycles 100 s to 25 or 100 ppm of ammonia followed by air purging at room temperature. The obtained results are given in Fig. 3, Fig. S4 and Fig. S5 (Supplementary information). A sharp increase in electrical resistance was observed when the SBS/PANI fibers were exposed to ammonia gas (Fig. 3). Responses as large as ΔR/R0 = 5.8 ± 0.3, 2.2 ± 0.2, 2.0 ± 0.1, and 2.0 ± 0.2 were observed at 25 ppm ammonia for SBS/ PANI0.01M fibers, SBS/PANI0.03M fibers, SBS/PANI0.05M fibers, and SBS/ PANI0.1M fibers, respectively (Fig. 3c). Therefore, as expected, SBS/ PANI0.01M fibers with PANI nano-layers consisting of interconnected nanofibers showed the highest response (ΔR/R0 = 5.8 ± 0.3 at 25 ppm and ΔR/R0 = 16.4 ± 0.8 at 100 ppm) to ammonia due to their high surface areas, small nanofiber diameter, and porous structure. As expected, with higher analyte concentration the response intensity increases. For applicability, Fig. 3 also indicates that the responses of SBS/ 89
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Fig. 3. Response of SBS/PANI0.01M fibers under cyclic exposure to (a) 25 and (b) 100 ppm of ammonia. (c) maximum response for the SBS/PANI fibers to 25 and 100 ppm ammonia in dependence on the composition. (d) Response time (averaged over at least three cycles) of SBS/PANI fibers fabricated at various aniline concentration during exposure of 25 and 100 ppm of ammonia.
Fig. 4. Response of the SBS/PANI0.01M fiber to ammonia (100 ppm) and different diluted organic vapors (concentration = 1%). Fig. 6. Comparison of sensitivity and response time of different PANI-based sensors for ammonia at 100 ppm.
reversible, that is, the doped PANI can be turned into its undoped state by chemical reactions. When PANI was exposed to ammonia gas, ammonia molecules diffused into PANI layer and took up protons from PANI, forming a more energetically favorable ammonium (NH4+) and the emeraldine base. This resulted in a decreased number of charge carriers, i.e., decreasing the doping level of PANI. As a result, the resistance of the PANI sharply increased [25]. This process is reversible, when the PANI sensor is exposed to air, the ammonium ion decomposes into ammonia and H+, then the H+ is captured by PANI, which restore its initial doping level and cause an increase in the sensor conductivity, approaching the initial resistance [17].
3.4. Mechanical reliability of SBS/PANI composite fiber Excellent mechanical reliability is expected to be one of the most important properties of fiber-based sensors for their real applications in flexible electronics. Fig. 8a shows the representative strain-stress curves of pure SBS fiber and SBS/PANI composite fiber. The fibers exhibited similar tensile behaviors as elastomeric materials. Good deformation capabilities and a maximum strain of up to 1000% is shown by the fibers, demonstrating their excellent flexibility. To evaluate the Fig. 5. (a) Response of the SBS/PANI0.01M fiber exposed to ammonia gas at different concentrations. (b) The sensor responses as a function of ammonia gas concentration from 10 ppm to 100 ppm.
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micro-injector, a few seconds later, the LED light turned on, and the buzzer gave an alarm (see Video S2 and Figure S7), demonstrating the practical applications of the developed SBS/PANI fiber based ammonia sensor for flexible electronics. A very important point to note is that according to the Chinese Indoor Air Quality Standard GB/T18883-2002 the maximum allowable concentration of ammonia is 0.2 ppm. When ammonia concentration reaches ∼5.3 ppm, it can be smelled. However, a person wearing the gas mask will not smell ammonia at this concentration. Therefore, this makes our fiber sensor applicable, as it would easily detect ammonia concentration of at least 1 ppm under masking conditions of the wearer. Fig. 7. Reaction mechanism in the ammonia gas sensing process.
4. Conclusions electrical reliability of the SBS/PANI composite fiber under bending deformations, a cyclic bending-straightening test was conducted using a homemade two-point bending device (see Video S1). The results of bending testing show that the resistance remains almost unchanged even after 1000 bending cycles, demonstrating the excellent electrical stability of the SBS/PANI fiber (Fig. 8b). Sensing behaviors in both straight and bent configurations were investigated during exposure to 25 ppm ammonia gas at room temperature (Fig. 8c). It is clearly observed that twisted SBS/PANI0.01M fiber shows a similar response to that of straight SBS/PANI0.01M fiber owing to the bendable and flexible natures of SBS fiber. These results indicate that the fibers had excellent mechanical reliability.
In conclusion, we reported a gas sensor based on a flexible SBS/ PANI fiber, fabricated through a facile process, which demonstrated superior performance for detection of ammonia gas. Owing to the high surface areas, small diameters, and porous structure, the response sensitivity of interconnected nanofibers was significantly better than those of nano-layers with irregular particles. The flexible SBS/ PANI0.01M fiber revealed several remarkable features including high sensitivity (5.8 for 25 ppm, 16.4 for 100 ppm), fast response (≤13 s), good reproducibility, ultra-low detection limit (0.1 ppm) as well as excellent mechanical reliability. Furthermore, we developed a flexible gas sensing and alarm system based on SBS/PANI fibers, demonstrating its tremendous potential applications in flexible electronics.
3.5. Development of a flexible SBS/PANI fiber based sensor for detection of ammonia gas
Acknowledgements
The SBS/PANI0.01M fiber was integrated into a commercial respirator and connected with electronic components to develop an ammonia gas sensor device with a LED light and a buzzer alarm (Fig. 9). The block diagram of the sensor circuit and optical image of the circuit board are shown in Fig. S6. The developed ammonia gas sensing respirator was put on a head model and placed in a 40 L chamber. Subsequently, 1 mL ammonia gas was injected into the chamber using
This work was supported by Shanghai Fundamental Research Projects (Project No. 16JC1400701), Program for Changjiang Scholars and Innovative Research Team in University (IRT16R13) and Opening Project of State Key Laboratory of for Modification of Chemical Fibers and Polymer Materials, Donghua University (Contract Number: LK1607). J.P. appreciates the support by the SKLPMF Visiting Scholar Funding Project Number LK1607. Fig. 8. (a) The representative stress-strain curves of pure SBS fiber and SBS/PANI fiber. (b) Resistance stability of SBS/PANI composite fiber during bending for 1000 times. (c) Gas response of straight and twisted SBS/PANI0.01M fiber exposed to ammonia at 25 ppm. (d) Photo of the straight and twisted test geometry.
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Fig. 9. Optical images of (a) SBS/PANI fiber and (b) commercial respirator with ammonia gas sensor.
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